The present invention generally relates to image sensor driving circuits, and more particularly to an image sensor driving circuit for use in a facsimile machine and the like which employ coplanar photoelectric conversion elements.
Generally, an apparatus for reading a document (image) by use of a coplanar image sensor comprises a plurality of photodetector elements arranged in a line. The photodetector elements are divided into photodetector element groups, and each photodetector element group is connected to a common first electrode. On the other hand, corresponding ones of the photodetector elements from each photodetector element group are connected to an individual second electrode through a matrix multilevel inter-connection.
FIG. 1 shows an example of the conventional image sensor driving circuit. The image sensor driving circuit comprises a control logic circuit 1, an image sensor 2 comprising a plurality of photodetector elements, a voltage applying circuit 3, and a current-to-voltage converting circuit 4. A voltage for reading information from the document (image) is sequentially applied to the image sensor 2 from the voltage applying circuit 3 under the control of the control logic circuit 1. The photodetector elements of the image sensor 2 receive a quantity of light dependent on the information optically read from the document, and supply photocurrents to the individual electrodes when applied with the voltage from the voltage applying circuit 3. The current-to-voltage converting circuit 4 sequentially converts the photocurrents into a detection voltage indicative of the read information.
FIG. 2 shows a cross sectional view of the photodetector element. The photodetector element generally comprises a substrate 10, a semiconductor layer 11 made of an optical semiconductor such as amorphous silicon having photoelectric conversion effect and made to a predetermined size by an etching, for example, metal electrodes 12 and 13 for obtaining a photocurrent, and highly doped portions 14 and 15 for making ohmic contact between the respective metal electrodes 12 and 13 and the semiconductor layer 11. For example, the metal electrode 12 is connected to the common first electrode described before, and the metal electrode 13 is connected to the individual second electrode.
In the conventional image sensor driving circuit, the voltage applied to the photodetector elements of the image sensor 2 from the voltage applying circuit 3 has a fixed polarity such as +12 V. But according to experiments conducted by the present inventors, a dark current Id from the photodetector element increases substantially with the voltage applying time when the voltage having the fixed polarity is applied to the photodetector element. In FIG. 3, a characteristic I shows a voltage applying time versus dark current characteristic obtained in the conventional image sensor driving circuit. Due to the large increase in the dark current Id with the voltage applying time, a light/dark ratio Il/Id between a photocurrent from the photodetector element which receives light and the dark current Id from the photodetector element which receives no light greatly decreases with the voltage applying time as indicated by a characteristic II in FIG. 4. Hence, in the conventional image sensor driving circuit, there is a problem in that the light/dark ratio Il/Id which influences the reading accuracy greatly deteriorates with the voltage applying time.
It is thought that the reason for the increase in the dark current Id occurring when the voltage having the fixed polarity is continuously applied to the photodetector element is due to the fact that some kind of positive (or negative) charge is induced at an interface between the metal electrode 12 (or 13) and the highly doped portion 14 (or 15), or at an interface between the highly doped portion 14 (or 15) and the semiconductor layer 11. It is thought that the induced charge is trapped at the interface, and a channel through which a current flows is formed between the two metal electrodes 12 and 13.
After the dark current Id increases, the dark current Id will not decrease at once to the original state in which the dark current Id is small even when the voltage is no longer applied to the photodetector element. From a time when the application of the voltage to the photodetector element is stopped, it takes time in order of several tens of hours for the dark current Id to decrease to the original state. In the facsimile machine or the like, the document is read continuously and the dark current Id will not decrease to the original state because the application of the voltage to the photodetector element would ordinarily not be stopped for a long period of time while in use.